The invention relates to a method for setting a supercharged internal combustion engine having exhaust-gas recirculation.
In order to reduce exhaust-gas emissions, internal combustion engines were previously provided with exhaust-gas recirculation, by way of which some of the exhaust gas generated is led out of the exhaust tract back into the intake tract of the internal combustion engine. This method has proved effective, even in supercharged internal combustion engines having an exhaust-gas turbine driven by exhaust gases which are under increased exhaust-gas back pressure and actuating a compressor for compressing fresh intake air to an increased boost pressure. In supercharged internal combustion engines of this type, there is the problem that, if the recirculation line branches off upstream of the exhaust-gas turbine, exhaust-gas recirculation is possible only at operating points at which the exhaust-gas back pressure exceeds the boost pressure. However, in this case, a positive scavenging gradient with a higher pressure at the cylinder inlet than at the cylinder outlet cannot be built up.
In order to overcome this problem, it is proposed, according to German Patent Document DE 195 21 573 A1, (corresponding to U.S. Pat. No. 5,682,746) to arrange between the exhaust tract and the intake tract a non-return valve which is opened counter to a return force by means of the exhaust-gas back pressure. This ensures that the connection between the exhaust tract and the intake tract is opened only at operating points with an exhaust-gas back pressure exceeding the boost pressure and exhaust gas can flow in the direction of the intake tract as a result of the positive pressure gradient.
The disadvantage of this version is that exhaust-gas recirculation is restricted to the operating points with an exhaust-gas back pressure exceeding the boost pressure. Continuous exhaust-gas recirculation is not possible, so that the combustion air supplied to the combustion spaces contains a permanently changing oxygen content, thus making clean, low-pollutant combustion more difficult.
The present invention provides exhaust-gas recirculation for an internal combustion engine in a way adapted to each operating point, expediently as a function of the load.
According to the method of the present invention, there is provision for the exhaust gas of only some of the cylinders of the internal combustion engine, and, in particular, of only a single cylinder, to be recirculated into the intake tract. The cylinders involved in exhaust-gas recirculation are designated as exhaust-gas dispenser cylinders. By the exhaust gas being removed completely from the dispenser cylinder or dispenser cylinders, the remaining cylinders not involved in exhaust-gas recirculation can be operated with a positive scavenging gradient in which the boost pressure exceeds the exhaust-gas back pressure. As a result, the volumetric recirculation rate corresponding to the ratio of the recirculated exhaust-gas volume flow to the total exhaust-gas volume flow, remains constant over the entire characteristic map of the internal combustion engine under normal conditions with the same injection of fuel into all the cylinders. This occurs because the volumetric recirculation rate is calculated according to the ratio of the number of dispenser cylinders to the total number of cylinders, and this ratio is not changed because only the exhaust gas of the dispenser cylinders is fed into recirculation.
So that the exhaust-gas recirculation rate can be set as a function of the operating point, there is provision for manipulating the injection of fuel into the combustion spaces of in the dispenser cylinders. A doubling of the injection quantity produces double the quantity of exhaust gas in the dispenser cylinder and, correspondingly, a halving of the injection quantity likewise reduces the exhaust gas to half. The injection of fuel into the combustion spaces of the dispenser cylinders takes place independently of injection into the remaining cylinders and is set, in particular, as a function of the load, so that, in contrast to the prior art, a variably settable exhaust-gas recirculation rate can be provided, even when one exhaust-gas dispenser cylinder is used. The exhaust-gas recirculation rate can thereby be set optimally in terms of minimized exhaust-gas emission and additional boundary conditions, such as low-noise running and a high power output in the full-load range.
In a preferred embodiment, injection of fuel into the combustion spaces of the exhaust-gas dispenser cylinders takes place in dependence on the load, in that the exhaust-gas recirculation rate follows a predetermined function which can be produced, in particular, in dependence on the load and not defined volumetrically, but as a function of the CO2 emission. In this case, the exhaust-gas recirculation rate is calculated from the ratio of the carbon dioxide fraction in the intake tract to the carbon dioxide fraction in the exhaust tract. The injection of fuel into the dispenser cylinders is carried out in a regulated manner in that the exhaust-gas recirculation rate follows the load-dependent predetermined profile.
The preferred profile predetermined for the exhaust-gas recirculation rate is a function which in the idling mode assumes a first minimum, in the part-load mode rises to a maximum and in the full-load mode falls to a second minimum, with the full-load minimum expediently being lower than the idling minimum. Accordingly, no or relatively little exhaust gas is recirculated into the intake tract both in the idling mode and in the full-load mode, whereas a relatively large exhaust-gas quantity is fed into the intake tract again in the part-load range. The exhaust-gas recirculation rate to be predetermined is expediently a continuous function, in particular a function which is continuous into the first derivation.
During idling, it is recommended to set the exhaust-gas recirculation rate according to the ratio of the number of dispenser cylinders to the total number of all the cylinders, this being equivalent to identical fuel injection for the dispenser cylinders and for the remaining cylinders. Uniform vibration distribution and low-noise running are thereby achieved in the idling mode or in the no-load mode.
Under full load, it may be advisable to set the exhaust-gas recirculation rate to half the value of the ratio of the number of dispenser cylinders to the total number of all the cylinders by the injection quantity for the dispenser cylinders being reduced to half the injection quantity of the remaining cylinders. The reduction in the exhaust-gas recirculation rate under full load improves the emission behaviour and increases engine power, possible non-uniformities in running due to the unequal injection of fuel into the dispenser cylinders and the remaining cylinders because of the increased vibration level in the full-load range being negligible since they cannot be detected or can be only slightly detected subjectively.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.